Sushi domains are commonly involved in protein-protein interactions. Sushi domain containing protein 4 (SUSD4) was a hypothetical cell surface protein composed of four continuous Sushi domains, and is highly conserved among different species ( >95% between human and mouse). In pilot studies we cloned, expressed, purified and refolded the sushi domains of human SUSD4. Using the refolded recombinant SUSD4 protein, we raised antibodies in chickens. We found that SUSD4 is only detectable in murine eyes, brains, spinal cords and testes, and in eyes its expression is restricted to photoreceptor outer segments. In vitro function assays, recombinant SUSD4 protein augments the alternative but not the classical pathway of complement activation, and in vivo studies zebrafish in which the SUSD4 expression is knocked down by Morpholinos exhibit developmental and locomotive disorders including abnormal eye development. In addition, previous clinical studies have connected deletions of SUSD4 gene with Fryn's syndrome and Autism in which many patients demonstrate abnormal electroretinograms (autism) and develop ocular abnormalities including anophthalmia and retinal dysplasia (Fryns syndrome). All these data, taken together, strongly argue that SUSD4 could be integrally involved in retinal development and/or function. To validate this hypothesis, we will study the role of SUSD4 in retinal development and function by combining expertise from 3 laboratories. In Aim 1, we will identify binding partners of SUSD4 using a newly developed, eye specific cDNA phage display system. After this, we will analyze the identified binding proteins by bioinformatics to explore potential roles of SUSD4 in retinal development and/or functions. In Aim 2, we will directly study the role of SUSD4 in retinal development in mice. We first will generate a photoreceptor-specific SUSD4 knockout mouse by developing a loxp sites-flanked SUSD4 gene targeted mouse, and then crossing this mouse with a photoreceptor specific Cre recombinase transgenic mouse. We will assess photoreceptors histologically and functionally in the resultant photoreceptor-specific SUSD4 deficient mice. In Aim 3, we will study the role of SUSD4 in retinal function. We will inactivate SUSD4 gene in developed retinas of adult SUSD4 conditional targeted mice that will be developed in Aim 2 by locally introducing Cre-recombinase, then examine photoreceptor structure and function using the same assays described in Aim 2. These studies will provide insights into the role of SUSD4, a gene that could be integrally involved in neurological and visual functions. The screening/validation of SUSD4 binding partners and the development of conditional SUSD4 knockouts will open new avenues of research on physiological roles of SUSD4, a new gene which has been implicated in the pathogenesis of autism and Fryns syndrome. PUBLIC HEALTH RELEVANCE: SUSD4 is a gene with unknown function and could be integrally involved in autism and Fryns syndrome in which patients develop retinal disorders. In this application, we will investigate the role of SUSD4 in retinal development and function. These results would provide insights into physiological roles of SUSD4 and could help to develop novel diagnostic and therapeutic strategies for autism and Fryns syndrome.